Polyolefins such as polyethylene and polypropylene and other polymers may be prepared by particle form polymerization, also referred to as slurry polymerization. In this technique, feed materials such as monomer and catalyst are introduced to a reaction zone, and a fluid slurry comprising solid polyolefin particles in a liquid medium is formed in the reaction zone.
In continuous loop reactors, the various feed materials may be introduced to the loop reaction zone in various ways. For example, the monomer and catalyst may be mixed with varying amounts of diluent prior to introduction to the loop reaction zone. In the loop reaction zone, the monomer and catalyst become dispersed in the fluid slurry. The fluid slurry circulates through the loop reaction zone, and the monomer reacts at the catalyst in a polymerization reaction. The polymerization reaction yields solid polymer particles in a liquid medium, for example, a liquid diluent used to form the fluid slurry.
In order to recover the polymer from a particle form polymerization process, it is necessary to separate the polymer solids from the liquid diluent that make up the effluent slurry withdrawn from the reactor. Typical separation systems include a reduction in pressure so that the liquid diluent vaporizes. The vaporized diluent exits at a top portion of a flash tank. The polymer remains solid, and is recovered through a bottom portion of the flash zone. Other polymer recovery processes utilize multiple stage flash systems. For example, a first intermediate pressure flash zone and a second lower pressure flash zone may be employed. The temperature and the pressure in the first flash zone are such that a major amount of the diluent will be vaporized and that vapor can be condensed without compression by heat exchange with a fluid having a temperature, for example, in the range of from about 40° F. to about 130° F. The polymer particles from the first flash step are then subjected to a lower pressure flash step to vaporize additional remaining diluent.
In a polymer recovery system utilizing one or more flash chambers, it has been common to have some diluent entrained in the polymer leaving the flash chamber(s). It is desirable, however, to obtain polymer product substantially free of diluent. If the polymer leaving the flash chamber contains an excess amount of diluent, the polymer product can become tacky, and then may plug up lines and cause restrictions. Furthermore, in many polymer production processes, polymer solids after the flash chamber are subjected to further processing to remove residual and entrained diluent. Examples of such processing include purge zones, conveyer dryers, and other drying systems known in the art.
One method for removing additional amounts of diluent after the flash chamber involves passing the polymer solids through a purge zone, wherein a non-combustible gas is used to remove the diluent.
Within a typical recovery system, when passing polymer from a higher pressure zone to a lower pressure zone, it is important to maintain a pressure seal between the zones. Otherwise, the pressure will equilibrate between the two zones, or diluent or other material from the higher pressure zone could pass into the next zone, or material from the lower pressure zone could pass up to the higher pressure zone. It is desirable to minimize the amount of gaseous or liquid dilient being passed to a purge zone to reduce the effort to purge such diluent and to ensure the production as an end-product of polymer that is essentially free of such diluent. Also, because at least a portion of the flash gas from the flash zone is often recycled to the reactor, it is desirable to keep purge gas from entering the flash zone.
A technique for maintaining a pressure seal between a higher pressure zone and a lower pressure zone includes the use of a “fluff chamber” or “surge vessel” between the zones. The fluff chamber is generally operated at a normal level of 75 to 85 percent of its capacity of polymer solids. This level is desired to minimize the amount of hydrocarbon gas that would be in the space above the polymer solids.
In association with a fluff chamber, valves are used to control flow of polymer solids (often called “fluff” or “flakes” or “powder”) into and out of the fluff chamber in a batch-type process. For example, in some recovery systems, when the fluff chamber inlet valve is open, polymer solids from the flash chamber or other higher pressure tank pass to the fluff chamber, and the fluff chamber outlet valve is closed to maintain a pressure seal. When the fluff chamber outlet valve is open, polymer solids pass out of the fluff chamber, and the pressure inside the fluff chamber is released, but the fluff chamber inlet valve is closed to maintain a pressure seal between the flash zone and the fluff chamber. In this scheme, the fluff chamber inlet and outlet valves are not open simultaneously. By keeping one of the valves always closed, the loss of high pressure diluent into a lower pressure downstream vessel is prevented.
As polymerization reactors get larger and production rates increase, the fluff chamber valves must also get larger and/or must cycle more often, presenting cost and maintenance concerns.